From what I understand the issue is that the quantity field is not accepting the greater number of digits you want it to accept . Can you confirm if the same error is happening when you order a cart or only while changing .Then check with multiple user ID to ensure this is not speific to User settings in SU01d .
I am not sure exactly but few suggestions if they might be helpful
Check SU01d for the user ID and see the decimal notation used Check this with the system default and see if its the same .
Then it may be due to price changes due to quantity changes . Check the note
1595423 - Price with more than 13 digits
Let me know your comments